The invention relates to channel estimation for orthogonal frequency division multiplexing (OFDM) systems and more particularly to channel estimation for OFDM systems having a plurality of transmitters.
OFDM multi-carrier modulation is used in many communication systems and has recently become increasingly popular because it provides a substantial reduction in equalization complexity compared to classical modulation techniques. For demodulating OFDM-modulated data in the presence of substantial time variations of the transmission channel, knowledge of the transmission channel frequency response is required. Such knowledge is obtained by channel estimation. Pilot-symbol aided channel estimation is based on periodically inserting known symbols, termed pilot symbols, in the transmitted data sequence. As in OFDM systems channel variations are in two dimensions, pilots are typically inserted in the time-frequency grid. The channel response can then be reconstructed via interpolation by exploiting the correlation of the channel in time and frequency. Interpolation in the time and frequency domain must comply with the two-dimensional sampling theorem.
The interpolation in time is bandwidth-limited by the time-variant behavior of the transmission channel. As these channel time variations are produced by the receiver's mobility (speed), pilot-based channel estimation is limited in terms of a maximum tolerable Doppler spread Fd. On the other hand, the interpolation in frequency is bandwidth-limited by the length of the channel impulse response (CIR) of the transmission channel, which is dependent on the multi-path propagation scenario exhibited in the transmission channel. Thus, pilot-based channel estimation is also limited in terms of a maximum tolerable multi-path delay spread Td.
As conventional pilot-based channel estimation can not tolerate under-sampling of channel variations in any direction (time, frequency), the channel estimation performance degrades when high Doppler spread Fd and large multi-path delay spread Td occur simultaneously. Using a “denser” pilot location pattern in the time-frequency grid would enhance the maximum tolerable Doppler spread Fd and the maximum tolerable multi-path delay spread Td. However, increasing the number of pilot symbols per time or frequency interval reduces the capacity of the transmission channel.
In view of the foregoing, it is desirable to provide for a high channel estimation performance over a wide variety of channel conditions.
For these and other reasons, there is a need for the present invention.